You’ll first encounter Guardian Arkveld in Chapter 3, during “The Faithful Keepers” main mission. The definitive fight won’t take place until the “Broken Chains” mission. As such, all you need to do to unlock the monster is progress through the main questline.

You get Monster Hardbone by killing the Large Monster Arkveld in Monster Hunter Wilds. Not to be confused with its Low Rank variant, Guardian Arkveld, the plainly named Arkveld is located all over the map in the Plains, Forest, Basin, Cliffs, and Wyveria regions.

Guardian Arkveld is the primary target of the Guild's investigations in the Forbidden Land in Monster Hunter Wilds. After tracking it through the region's various biomes, you've finally found the artificial monster deep in the Ruins of Wyveria.